Import Brushset add_on
The Import BrushSet addon is not an official addon that is included with blender releases. While it may be added to future versions, you will need to download and install it manually until then.

The BrushSet addon can be found in the Blender Add-ons Contrib repo and as it is a single file, you can easily download the one file and install it. Save the file from this link, click the Install Add-On button and select the downloaded file.